It is recognized that the size of a bubble blown from a film of "soap" solution depends to a major extent on the diameter of the opening upon the margin of which the bubble is formed. However, the larger the opening, the greater the energy required to form the film into a bubble of maximum diameter. Thus, a child will encounter greater difficulty because the capacity of the child's lungs is limited. For this reason, the production of larger bubbles may be said to be dependent on the child's lung-power.